The present invention relates to a test fixture for an integrated circuit IC device and more specifically to a test fixture adapted for use with high pin count surface mounted devices.
Surface mount technology SMT is increasing being used in the electronic industry to produce high pin count, small dimensioned IC devices. The pins of the SMT device are soldered directly to conductive runs on a circuit board. The distance between pin centers for these types of devices are generally in the range of 0.025 inches producing a much higher pin density than in previous types of IC devices. This higher pin density makes it much more difficult to probe and test these devices. For example, the 80386 microprocessor chip, manufactured by Intel Corp., has one hundred pins positioned around the periphery of the chip on 0.025 inch centers.
During the development and testing of new IC devices, such as the 80386 microprocessor, in-circuit emulation of the device is performed with the chip removed from the circuit. Removing a SMT device from a circuit board can easily produce damage to the device or the circuit board. For this reason, chip carriers are used for mounting the device on a circuit board. An example of such a chip carrier for a 80386 microprocessor is manufactured by Minnesota, Mining & Manufacturing, 3M, under Part No. 2-0100-07243-003-018-000. The chip carrier mounts on the circuit board and the chip device is placed within a recess formed in the carrier. A coverplate is placed over the chip and snap fitted into place to secure the device in the carrier. Individual pins of the device mate with coextensive conductor wires that are soldered to the circuit board. During in-circuit emulation of the device, the IC device is removed from the 3M chip carrier and a in-circuit-emulation test head assembly, developed by the assignee of the present invention and sold by Intel, Corp. under the name Hinge Cable Assembly with Part No. 457451-001, is used to target the 3M chip carrier. The test head assembly has a flex-circuit cable with a coverplate adapter on one end and a probe head on the other. The flex-circuit cable is secured in the coverplate adapter so that electrically conductive pads formed on the flex-circuit cable are aligned with the corresponding conductor wires of the chip carrier. The coverplate adapter replaces the coverplate of the chip carrier socket. The probe head has contacting points thereon in a pattern corresponding to the pin out pattern of the displaced IC that allows probing without the attendant risk of shorting pins. In use, the test head assembly couples externally-generated signals into the chip carrier socket as though the chip were present from an instrument which provide outputs to simulate operations of the displaced IC. The Intel embodiment targets a system in which a socketed device is removed.
Another disadvantage of using chip carriers, such as the 3M device, is the high profile that the carrier has on the circuit board. Elimination of the chip carriers would allow closer spacing of circuit boards in addition to the attendant cost saving with a reduction in parts.
At present no test fixture exists for probing an in-place, operating device. What is needed is a test fixture for use with SMT IC devices that provides in-circuit monitoring and/or emulation with the IC device in place.